Liquid crystal display devices have come to be used in watches, calculators, various home electric appliances, measurement instruments, automobile panels, word processors, electronic organizers, printers, computers, televisions, and other applications. Representative examples of the liquid crystal display mode include twisted nematic (TN) mode, super twisted nematic (STN) mode, dynamic scattering (DS) mode, guest/host (GH) mode, in-plane-switching (IPS) mode, optically compensated bend (OCB) mode, electrically controlled birefringence (ECB) mode, vertical alignment (VA) mode, color super homeotropic (CSH) mode, and ferroelectric liquid crystals (FLC). As for the driving mode, multiplex driving has become popular replacing the conventional static driving, and the mainstream mode is a simple matrix mode, and recently, an active matrix (AM) mode in which driving is performed by using thin film transistors (TFTs) or thin film diodes (TFDs).
A typical color liquid crystal display device is, as shown in FIG. 1, configured by two substrates (1) each having an alignment film (4), a transparent electrode layer (3a) serving as a common electrode and a color filter layer (2) that are disposed between one of the substrates and the alignment film of this substrate, a pixel electrode layer (3b) disposed between the other substrate and the alignment film of this other substrate, and a liquid crystal layer (5) interposed between the alignment films of the substrates arranged so that the alignment films oppose each other.
The color filter layer is constituted by a color filter that includes a black matrix, a red color layer (R), a green color layer (G), a blue color layer (B), and, if needed, a yellow color layer (Y).
A liquid crystal material constituting a liquid crystal layer has been subject to strict impurity control since impurities remaining in the material will significantly affect electrical properties of display devices. As for the material of an alignment film, the alignment film comes into direct contact with the liquid crystal layer and thus it is known that the impurities remaining in the alignment film will migrate to the liquid crystal layer and affect the electrical properties of the liquid crystal layer. Studies are now being conducted on the properties of liquid crystal display devices affected by impurities in the alignment film materials.
As for materials, such as organic pigments, used in color filter layers, it is possible that the liquid crystal layers would be affected by the impurities contained in the color filters as with the alignment film materials. However, it has been believed that because an alignment film and a transparent electrode are disposed between a color filter layer and a liquid crystal layer, the direct effect on the liquid crystal layer is significantly less than that of the alignment film materials. However, an alignment film usually has a thickness as small as 0.1 μm or less and the thickness of the transparent electrode is usually 0.5 μm or less even for a color-filter-layer-side common electrode that has been made thick to increase the conductivity. Accordingly, it is not proper to assume that a color filter layer and a liquid crystal layer are completely isolated from each other and it is possible that display defects such as missing dots, alignment variation, and ghosting would occur due to a decrease in voltage holding ratio (VHR) and an increase in ion density (ID) of the liquid crystal layer caused by impurities migrating from the color filter layer through the alignment film and the transparent electrode.
Techniques for overcoming display defects caused by impurities in the pigments constituting color filters have been investigated, which include a technique of controlling elution of impurities into liquid crystals by using a pigment that contains a particular fraction or less of an extract from the pigment with ethyl formate (PTL 1) and a technique of controlling elution of impurities into liquid crystals by specifying the pigment used in a blue color layer (PTL 2). However, these techniques are not much different from simply decreasing the amount of impurities contained in the pigment and the improvements brought about thereby are insufficient for resolving display defects under current situations of advancement in pigment purification techniques.
There have also been disclosed a technique that focuses on the relationship between a liquid crystal composition and organic impurities contained in a color filter, in which insolubility of organic impurities in a liquid crystal layer is indicated by a hydrophobicity parameter of liquid crystal molecules contained in the liquid crystal layer and this parameter is controlled to a particular level or higher, and a technique of preparing a liquid crystal composition that contains a particular fraction or more of a liquid crystal compound having an —OCF3 group in a liquid crystal molecule terminus since this hydrophobicity parameter and the —OCF3 group in a liquid crystal molecule terminus are correlated to each other (PTL 3). However, in the disclosure of the cited document, the essence of the invention is to reduce the influence of the impurities in the pigments on the liquid crystal layer; and the direct relationship between the overall structure of the liquid crystal material and the pigments used in the color filter has not been investigated.
It has also been disclosed that the voltage holding ratio (VHR) can be enhanced by using a pigment washed with deionized water until the electrical conductivity of the deionized water filtrate after the washing treatment is 20 μS/cm or less; however, the electrical conductivity of the pigment itself is not mentioned and the voltage holding ratio is only about 95% (PTL 4). This is not sufficient for resolving display defects of recent advanced liquid crystal display elements.
It is known that the water soluble matter in and the electrical conductivity of a pigment affect the antirust effect of antirust paints and discharge properties of inkjet inks (PTL 5 and PTL 6); however, the effect of a combination of the water soluble matter and the electrical conductivity of the pigment and the structure of the liquid crystal material constituting the liquid crystal layer has not been known, and the problem of display defects in advanced liquid crystal display devices has not been resolved.